Capillary endothelial cells derived from human brain (HBCEC) synthesize prostaglandin D2 (PGD2) which can be stimulated, among other prostanoids, by endothelin 1 (ET-1). Exogenous PGD2 dose-dependently augmented the production of vasoconstrictive prostaglandin F2alpha (PGF2alpha), 9alpha- 11-beta prostaglandin F2 (9alpha-11-beta PGF2), and thromboxane B2 (TxB2) and vasodilatory prostaglandin E2 (PGE2) as well as cAMP by HBCEC. These results suggest that PGD2 may play a role in cerebral capillaries under physiologic and pathologic conditions.